Feedback is a common problem in hearing aids, especially in-the-ear ("ITE") type hearing aids, and adversely affects overall hearing aid operation and performance. The feedback limits the maximum usable gain of the hearing aid and degrades overall system response. Feedback in a hearing aid includes both acoustical and mechanical feedback. Acoustical feedback occurs when a portion of the output sound signal is provided back to the input of the hearing aid. The portion of the sound signal that travels back to the input will generally travel through a vent or through any acoustical leakage path that may exit between the hearing aid shell and the ear into which the hearing aid is inserted. Mechanical feedback is caused by the vibrations of the receiver being transmitted back to the microphone through the tubes and the walls of the hearing aid shell.
Prior attempts at solving the feedback problem have focused on feedback cancellation which includes estimating the feedback signal and subtracting it from the input signal provided to the microphone of the hearing aid. These prior attempts are discussed in James M. Katz, Feedback Cancellation in Hearing Aids: Results from a Computer Simulation, IEEE Transactions on Signal Processing, Vol. 39, No. 3, pp. 553-562, 1991. Prior feedback cancellation techniques may be divided into two areas.
The first area involves providing a fixed filter, with fixed filter coefficients or taps, that cannot adjust to changes in the acoustical feedback path. This solution has proven unsatisfactory because changes in the acoustical environment occur frequently and thus the fixed filter no longer accurately models the feedback path and cannot provide accurate results. For example, changes in the acoustical environment may occur when a telephone receiver is moved close to the aided ear or when a hand is brought to the hearing aid to adjust the volume control. Other examples of changes in the acoustical environment include when the hearing aid shifts or adjusts in the user's ear, which often occurs when the user is eating or talking. Thus, the fixed filter feedback cancellation technique has proven unsatisfactory in solving the feedback problem.
A second approach at solving the feedback problem using feedback cancellation involves estimating the effects of the feedback path using a digital adaptive system. The digital adaptive system updates the estimated feedback path whenever changes are detected in the feedback behavior. The criterion for determining when a change has occurred in the feedback behavior is the onset of oscillation. When oscillation is detected, the normal hearing aid processing is interrupted and stopped and a pseudorandom noise burst is injected into the feedback path and a set of digital filter coefficients are adjusted to update the estimate of the feedback path. The hearing aid is then returned to normal operation with the feedback cancellation filter using the updated filter coefficient or taps. This approach suffers from several problems and disadvantages. This noncontinuous estimation approach fails to detect small changes in the feedback path if these changes do not cause oscillation or instability. Another disadvantage includes a reduction in speech intelligibility as a result of the feedback cancellation being disabled while the feedback path is being modeled.